An oxide semiconductor material, due to its advantages of high carrier mobility, low preparation temperature, good electrical uniformity, being transparent to visible light, low cost and the like, is considered among the most suitable semiconductor active materials of a thin film transistor (TFT) for driving an organic light emitting diode (OLED). Currently, an oxide TFT has been widely applied to an array substrate of a flat panel display. The oxide semiconductor is so sensitive to light that it should be shield to prevent ambient light from affecting performance of TFT. Especially for a TFT of a top gate structure, an active layer is located relatively close to a substrate, so that it is prone to irradiation by an external light source below the panel, which results in degradation of the device performance.